The Hartmann mask and Scheiner Disk are simple devices for making focusing a lens easier. They require that you target is a point of light, such as a star or distant planet, therefore are no good for day-to-day use.
A Scheiner Disk is a mask with two holes, where as a Hartman mask has many holes (typically three).
The mask is like a lens cap with holes in. The holes are positioned in such a way that when out of focus, they cause multiple diffraction images as shown in the first image. As the lens is focused they merge into a single point, and then beyond focus they separate again.
The masks can use either a circular hole or a triangular hole. The triangular hole creates diffraction spikes which are easier to detect when in focus, but the circular holes will allow more light to enter the lens resulting in a clearer image. Once your telescope is in focus, the mask can be removed.
I haven't been in astronomy very long, but a Hartmann mask is one of the most useful accessories that I have used. It makes focusing so much easier and sharp images can be had in seconds. What makes it even better is that you can make one for free!
You could have a mixture of circles for light gathering and triangles for diffraction. I imagine this would create an interesting pattern and I will try it some time.
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